Application of ultrasound for medical diagnostic purposes is well known. However, the development of therapeutic applications of ultrasound is a relatively new and rapidly developing technology. Treatment by ultrasound has many advantages and it is generally acknowledged that there are fewer side effects compared with other therapeutic treatment techniques.
In order to cause a desired therapeutic effect, application of ultrasound requires ultrasound power in order of more than a magnitude higher than the one required for diagnostic purposes. Ultrasound is introduced into the treated subject with the help of an ultrasound transducer. An ultrasound transducer is a device that converts electric energy into ultrasound energy or ultrasound waves. Usually, this term refers to piezoelectric transducers that convert electrical energy into ultrasound. Accordingly, advances in transducer technology play an important role in this technological area.
Specific characteristics of a high power ultrasound transducer relate to the ability of providing and sustaining without damage high peak power with high duty cycles; focusing ultrasound and focal spot location control; access to deeper layers of treated tissue, and providing a feedback to control equipment enabling the operator of changing treatment parameters.
Typical high power transducers used for therapeutic treatment are composed of piezoelectric material plates, having conducting electrodes on both sides and driven by an alternating voltage (alternating current—AC) electrical power generator. The typical operating frequency of these transducers is in the range from 100 kHz to 5 MHz. Usually, transducer side applied to the treatment location has an acoustic impedance matching element to compensate for the large difference between the transducer acoustic impedance and the treated subject acoustic impedance. The opposite side of the piezoelectric material is coupled with either ultrasound reflecting or absorbing material. Efficient use of the energy generated by high power ultrasound transducers is imperative and therefore absorbing backings are not used. The absorbing backing is usually replaced by a backing having large acoustic impedance mismatch with the piezoelectric ceramics that reflects most of the ultrasound energy, since such structure reduces ultrasound energy waste. The reflecting material can be one with acoustic impedance significantly different from that of the piezoelectric material.
Air is the best reflecting material; however, air cannot be used for high power transducers, where heat removal is a major problem. Piezoelectric ceramic must be provided with a way to remove heat efficiently and air does not possess proper thermal conductivity properties. Oil or solid material with high thermal conductivity are more frequently used for high power ultrasound transducers. The efficient heat removal requirement contradicts some of the solutions used for good ultrasound coupling.
Phased array transducers are more effective than conventional planar or curved piezoelectric transducers and they are typically used for high power ultrasound treatment applications. Phased array transducers are made by cutting the piezoelectric materials into individual piezoelectric elements—sometimes termed “pixels”, with each pixel having its own-wired connection to an allocated electrical driver. By controlling the phases of each of the electrical drivers, the ultrasound beam could be electronically scanned in the treated location. The phased array structure also has the advantage of reducing parasite oscillations mode compared to a single piece transducer.
Production and use of high-power phased array transducers operating at high peak power and relatively low frequencies pose a number of problems. Piezoelectric elements or pixel size and piezoelectric ceramics material thickness are in the range of a few millimeters. They are attached to the acoustic impedance matching plate with the help of adhesives or soldering, or potting of materials one on the other. The mechanical load caused by the ultrasound vibrations is maximal at the interface of the piezoelectric ceramics with the matching plate. At high peak power, the strength of bonding is not sufficient and the bonding is damaged, so the lifetime of the transducer is short. In case of over-driving the transducer, irreversible damage can occur. Soldering provides a stronger bond than gluing and because of this, instead of the bond, the ceramic piezoelectric material fails.
Electrodes soldered or glued to the piezoelectric ceramics tend to fail at high power. In some extreme cases, when indeed high ultrasound power is applied the voltage supplying wires may be cut by shear tension. Both direct gluing or soldering of conductive wires to the contacts of the piezoelectric ceramics of flexible printed circuits might fail at high power.
These and other problems are impeding faster development of the technology and should be partially or fully resolved.